The octapeptide. Phe-Leu-Phe-Gln-Arg-Phe-NH2 (F-8-F-NH2) was first detected by the antiserum raised against the molluscan cardioexcitatory peptide. Phe-Met-Arg-Phe-NH2, and subsequently isolated from bovine brain. F-8-F-NH2 injected intraventriculary can decrease the analgesic effect of Morphine. F-8-F-NH2 was found to be highly localized in substantia gelatinosa of dorsal spinal cords. In the study using in vitro and in vivo superfusion of spinal cords, we found that F-8-F-NH2 can be released by depolarizing concentration of KCI (in vitro) and also by substance P, the sensory transmitter (in vivo). This result further support our hypothesis that F-8-F-NH2 may have a modulatory role in antinociception. In rats, F-8-F-NH2 was found to be also highly concentrated in posterior lobes of pituitary glands. In searching for the role of this peptide in the pituitary, F-8-F-NH2 was found to be below the limit of detection in pituitary glands of Brattleboro rats, diabetes insipidus rats. The result indicates that Brattleboro rat may be an useful model to explore the function of pituitary F-8-F-NH2. Using Immunohistochemical technique, two strongly F-8-F-NH2 immunoreactive neuronal groups in rat brains were revealed. One of them was located in the periventricular hypothalamic area and another prominent cell group was found in the nucleus tratus solitarii. Whether these cell bodies project to pituitaries and spinal cord are under current investigation. The proposed course of this study is to investigate 1) function of pituitary F-8-F-NH2 using Brattleboro rats, 2) release of F-8-F- NH2 from the spinal cord and the effect of morphine on this release and 3) sources of F-8-F-NH2 immunoreactive nerve terminals in spinal cords and pituitaries using lesion techniques.